1. Field of the Invention
The present invention relates to an apparatus for communication devices, in particular and apparatus for generating a ringing voltage.
2. Description of the Related Art
Communications devices, such as modems, employ digital processing techniques to transmit data over a band-limited communications channel, such as the Public Switched Telephone Network (PSTN). Generally, to transmit data over the PSTN, a modem modulates the binary data and then transmits the data over the band-limited communications channel. A receiving modem is coupled to the band-limited communications channel, and receives the binary data and demodulates the binary data.
Generally, the modem is in an idle state until it is ready to transmit or receive data. When the modem is ready to transmit data to the PSTN, the modem begins a connect process. Similarly, when the modem needs to receive data from the PSTN, the modem goes through a call set-up process.
Modems are typically designed to operate on the PSTN. Several types of services are available on the PSTN, such as plain old telephone services (POTS), Integrated Services Digital Network (ISDN) services, or x Digital Subscriber Line (xDSL) services. The communication devices that are intended to operate on the PSTN, are generally subject to an interface standard. The interface standard for the United States is specified in EIA/TIA-496-A (1998). The standard establishes technical requirements for interfacing and connecting a communication device to the PSTN for purposes of data transmission. The communication device""s compliance with these requirements is generally intended to assure minimum acceptable compatibility with the PSTN.
As mentioned previously, the modem is in an idle state until it is ready to send or receive data. Typically, the modem while in the idle state is on-hook. Should another communication device wish to transmit data to the receiving modem, the PSTN will initiate a call setup by providing a ringing voltage to the receiving modem. The electrical characteristic of the ringing voltage is specified in the EIA/TIA standard. Once the receiving modem receives the ringing voltage from the PSTN, the receiving modem can either remain on-hook or go off-hook and begin receiving the data. Off-hook refers to an operating state of the communication device in which a communication link is enabled for voice communication, data communication or network signaling. On-hook refers to an operating state of the communication device in which a communication link is disabled.
Other networks, such as a PBX system, may not provide the necessary ringing voltage to the modem for call setup. Typically, the communication devices coupled to the PBX system generally use digital controls signals for the call setup process. These communication devices tend to be more complex, and therefore more expensive than those communication devices that use the ringing voltage for the call setup process.
Briefly, an apparatus provides a ringing voltage to an Analog Front End (AFE) of a communication device while the device is in an on-hook state. The apparatus provides the necessary circuitry for connecting a communication device, such as a modem or a telephone, with a telephone network, such as a PBX system. When the communication device is in the on-hook state, the apparatus can provide a ringing voltage to the AFE when the communication device is coupled to a telephone network that does not provide the ringing voltage. The apparatus may be incorporated into the communication device, or be a stand-alone device coupled to the communication device. This apparatus is used to provide a ringing voltage to the communication devices coupled to a telephone network that does not provide the ringing voltage.